anus:

The terminal end of the alimentary canal opening to the outside of the body through which feces are discharged; the excretory opening of the rectum, the final one or one and a half inches of the rectum often confused with the rectum itself; the outlet of the gastrointestinal tract; the excremental orifice. The anus comprises both involuntary and voluntary sphincter muscles which relax to allow defecation or penile insertion during anal-intercourse . See anus [SYN] for synonyms.